Microelectromechanical systems (MEMS) are miniature mechanical devices manufactured using the techniques developed by the semiconductor industry for integrated circuit fabrication. Such techniques generally involve depositing layers of material that form the device, selectively etching features in the layer to shape the device and removing certain layers, known as sacrificial layers, to release the device. Such techniques have been used, for example, to fabricate miniature electric motors as described in U.S. Pat. No. 5,043,043.
Silicon-on-insulator (SOI) techniques have been developed for fabricating MEMS devices. In SOI, an oxide layer is grown or deposited on a silicon wafer. A second silicon wafer is then bonded to the oxide layer, e.g. by fusion bonding. After bonding, the second silicon wafer is ground back and polished such that a thin layer of silicon is left attached to the oxide layer to form an SOI substrate. SOI substrates are particularly useful for MEMS devices where a moveable element formed from a silicon device layer is to be electrically insulated from an underlying support layer.
Recently, MEMS devices have been developed for optical switching. Such systems typically include an array of mechanically actuatable mirrors that deflect light from one optical fiber to another. Such MEMS optical switches are described, for example in U.S. Pat. No. 5,960,132. The mirrors are configured to translate or rotate into the path of the light from the fiber. Mirrors that rotate into the light path generally rotate about a substantially horizontal axis, i.e., they “flip up” from a horizontal position into a vertical position. MEMS mirrors of this type are usually actuated by magnetic interaction, electrostatic interaction, thermal actuation or some combination of these.
When the mirror is in the horizontal position, it rests against a substrate that forms a base. Often, the mirror is subject to electromechanical forces, sometimes referred to as “stiction” that cause the mirror to stick to the substrate and prevent the mirror from rotating. In addition, stiction forces can also prevent the mirror from being properly released from the substrate during manufacture. The mechanism by which stiction occurs can be divided into two stages: (a) mechanical collapse of the released portion of the microstructure to contact or move very close to the substrate and (b) adhesion of the released portion of the microstructure to the substrate. The microstructure's mechanical collapse can be initiated by high surface tension forces resulting from etchant rinse liquid trapped in the capillary-like spaces between the microstructure and the substrate, or by residual electric charges on the microstructure and/or the substrate. Several mechanisms have been proposed to explain the adhesion of the microstructure to the substrate, including solid bridging, liquid bridging, Van der Waals forces, and hydrogen bonding. Often the stuck part can be separated with increased force, but sometimes a permanent bond is formed after the initial contact.
A number of techniques have been developed to avoid stiction. One technique is to reduce the real contact area between the released portion of the microstructure and the underlying substrate either through nanoscale roughness intrinsic to one or both surfaces or through the formation of microscale standoffs in the form of bumps or “dimples” on the microstructure. However, such standoffs are difficult to fabricate, particularly when they are to be fabricated from the device layer of an SOI substrate. Consequently the stand-offs add an additional level of complexity to the fabrication of the MEMS device. The additional complexity increases the cost and reduces the yield of usable MEMS devices. Another group of stiction-inhibition techniques eliminates the source of surface tension between the released portion of microstructure and the substrate and prevents the microstructure's initial collapse by eliminating the gas-liquid interface. A third alternative procedure utilizes a self-assembled monolayer to reduce the surface energy. Often, a combination of two or more of these methods is required to eliminate the problem of stiction. All of these techniques add to the complexity and cost of the MEMS device.
Thus, there is a need in the art, for a simple, low-cost way of reducing stiction in MEMS devices.